Technical Field
The present disclosure relates to allocation mechanisms of radio resources to a transmitting user equipment to perform multiple direct sidelink transmissions during a sidelink period over a sidelink interface to one or more receiving user equipments. In this respect, the present disclosure defines methods for the allocation mechanism and also user equipments applying the allocation mechanism described herein.
Description of the Related Art
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM)-based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA)-based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall LTE architecture is shown in FIG. 1. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle-state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, or network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle-mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at the time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME, and it is also responsible for the generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 2, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective subcarriers. In LTE, the transmitted signal in each slot is described by a resource grid of NRBDLNscRB subcarriers and NsymbDL OFDM symbols. NRBDL is the number of resource blocks within the bandwidth. The quantity NRBDL depends on the downlink transmission bandwidth configured in the cell and shall fulfill NRBmin,DL≤RBDL≤RBmax,DL, where NRBmin,DL=6 and NRBmax,DL=110 are respectively the smallest and the largest downlink bandwidths, supported by the current version of the specification. NscRB is the number of subcarriers within one resource block. For normal cyclic prefix subframe structure, NscRB=12 and NsymbDL=7.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block (PRB) is defined as consecutive OFDM symbols in the time domain (e.g., 7 OFDM symbols) and consecutive subcarriers in the frequency domain as exemplified in FIG. 2 (e.g., 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following the time-frequency resources equivalent to the same consecutive subcarriers spanning a full subframe is called a “resource block pair”, or equivalent “RB pair” or “PRB pair”.
The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Similar assumptions for the component carrier structure will apply to later releases too.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g., to fulfill the requirements on IMT-Advanced.
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE may be in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the bandwidth of a component carrier does not exceed the supported bandwidth of an LTE Rel. 8/9 cell. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanisms (e.g., barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit on one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. An LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain (using the 3GPP LTE (Release 8/9) numerology).
It is possible to configure a 3GPP LTE-A (Release 10)-compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may currently not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers. In a typical TDD deployment the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not provide the same coverage.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time to preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g., TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC). Maximum five serving cells, including the PCell, can be configured for one UE.
The characteristics of the downlink and uplink PCell are:                For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only)        The downlink PCell cannot be de-activated, unlike SCells        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        Non-access stratum information is taken from the downlink PCell        PCell can only be changed with handover procedure (i.e., with security key change and RACH procedure)        PCell is used for transmission of PUCCH        The uplink PCell is used for transmission of Layer 1 uplink control information        From a UE viewpoint, each uplink resource only belongs to one serving cell        
The configuration and reconfiguration, as well as addition and removal, of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover). Each SCell is configured with a serving cell index, when the SCell is added to one UE; PCell has always the serving cell index 0.
When a user equipment is configured with carrier aggregation there is at least one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled on multiple component carriers simultaneously, but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI (Downlink Control Information) formats, called CIF.
A linking, established by RRC signaling, between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data, thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g., a subframe of 0.5 ms, onto which coded information bits are mapped. It should be noted that a subframe, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of subframes.
UL Scheduling Scheme for LTE
The uplink scheme in LTE allows for both scheduled access, i.e., controlled by eNB, and contention-based access.
In case of scheduled access, the UE is allocated by the eNB a certain frequency resource for a certain time (i.e., a time/frequency resource) for uplink data transmission. Some time/frequency resources can be allocated for contention-based access, within which the UEs can transmit without first being scheduled by the eNB. One scenario where UE is making a contention-based access is for example the random access, i.e., when UE is performing an initial access to a cell or for requesting uplink resources.
For the scheduled access the Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which UE(s) is (are) allowed to transmit,        which physical channel resources,        Transport format (Modulation Coding Scheme, MCS) to be used by the mobile terminal for the transmission        
The allocation information is then signaled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons this channel is called uplink grant channel in the following. Correspondingly, a scheduling grant message contains information which part of the frequency band the UE is allowed to use, the validity period of the grant, and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e., there are no “per UE per RB” grants). Therefore, the UE needs to distribute the allocated resources among the radio bearers according to some rules. Unlike in HSUPA, there is no UE-based transport format selection. The eNB decides the transport format based on some information, e.g., reported scheduling information and QoS info, and the UE has to follow the selected transport format. In HSUPA the Node B assigns the maximum uplink resource, and the UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared-channel access network for determining Quality of Service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management.                Starvation of low priority services should be avoided        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme        The UL reporting should allow fine granular buffer reports (e.g., per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent.        It should be possible to make clear QoS differentiation between services of different users        It should be possible to provide a minimum bit rate per radio bearer        
As can be seen from above list, one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.
Layer 1/Layer 2 Control Signaling
In order to inform the scheduled users about their allocation status, transport format, and other transmission-related information (e.g., HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length can be a multiple of the subframes. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI. Without loss of generality, the following assumes that a TTI is equivalent to one subframe.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which in most cases includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH. Furthermore, 3GPP Release 11 introduced an EPDCCH that fulfills basically the same function as the PDCCH, i.e., conveys L1/L2 control signaling, even though the detailed transmission methods are different from the PDCCH. Further details can be found in the current versions of 3GPP TS 36.211 and 36.213, incorporated herein by reference. Consequently, most items outlined in the background and the embodiments apply to PDCCH as well as EPDCCH, or other means of conveying L1/L2 control signals, unless specifically noted.
Generally, the information sent in the L1/L2 control signaling for assigning uplink or downlink radio resources (particularly LTE(-A) Release 10) can be categorized to the following items:                User identity, indicating the user that is allocated. This is typically included in the checksum by masking the CRC with the user identity;        Resource allocation information, indicating the resources (e.g., Resource Blocks, RBs) on which a user is allocated. Alternatively this information is termed resource block assignment (RBA). Note, that the number of RBs on which a user is allocated can be dynamic;        Carrier indicator, which is used if a control channel transmitted on a first carrier assigns resources that concern a second carrier, i.e., resources on a second carrier or resources related to a second carrier; (cross carrier scheduling);        Modulation and coding scheme that determines the employed modulation scheme and coding rate;        HARQ information, such as a new data indicator (NDI) and/or a redundancy version (RV) that is particularly useful in retransmissions of data packets or parts thereof;        Power control commands to adjust the transmit power of the assigned uplink data or control information transmission;        Reference signal information such as the applied cyclic shift and/or orthogonal cover code index, which are to be employed for transmission or reception of reference signals related to the assignment;        Uplink or downlink assignment index that is used to identify an order of assignments, which is particularly useful in TDD systems;        Hopping information, e.g., an indication whether and how to apply resource hopping in order to increase the frequency diversity;        CSI request, which is used to trigger the transmission of channel state information in an assigned resource; and        Multi-cluster information, which is a flag used to indicate and control whether the transmission occurs in a single cluster (contiguous set of RBs) or in multiple clusters (at least two non-contiguous sets of contiguous RBs). Multi-cluster allocation has been introduced by 3GPP LTE-(A) Release 10.        
It is to be noted that the above listing is non-exhaustive, and not all mentioned information items need to be present in each PDCCH transmission depending on the DCI format that is used.
Downlink control information occurs in several formats that differ in overall size and also in the information contained in their fields as mentioned above. The different DCI formats that are currently defined for LTE are as follows and described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (current version v12.4.0 available at http://www.3gpp.org and incorporated herein by reference). In addition, for further information regarding the DCI formats and the particular information that is transmitted in the DCI, please refer to the mentioned technical standard or to LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.                Format 0: DCI Format 0 is used for the transmission of resource grants for the PUSCH, using single-antenna port transmissions in uplink transmission mode 1 or 2.        Format 1: DCI Format 1 is used for the transmission of resource assignments for single codeword PDSCH transmissions (downlink transmission modes 1, 2 and 7).        Format 1A: DCI Format 1A is used for compact signaling of resource assignments for single codeword PDSCH transmissions, and for allocating a dedicated preamble signature to a mobile terminal for contention-free random access (for all transmissions modes).        Format 1B: DCI Format 1B is used for compact signaling of resource assignments for PDSCH transmissions using closed loop precoding with rank-1 transmission (downlink transmission mode 6). The information transmitted is the same as in Format 1A, but with the addition of an indicator of the precoding vector applied for the PDSCH transmission.        Format 1C: DCI Format 1C is used for very compact transmission of PDSCH assignments. When format 1C is used, the PDSCH transmission is constrained to using QPSK modulation. This is used, for example, for signaling paging messages and broadcast system information messages.        Format 1D: DCI Format 1D is used for compact signaling of resource assignments for PDSCH transmission using multi-user MIMO. The information transmitted is the same as in Format 1B, but instead of one of the bits of the precoding vector indicators, there is a single bit to indicate whether a power offset is applied to the data symbols. This feature is needed to show whether or not the transmission power is shared between two UEs. Future versions of LTE may extend this to the case of power sharing between larger numbers of UEs.        Format 2: DCI Format 2 is used for the transmission of resource assignments for PDSCH for closed-loop MIMO operation (transmission mode 4).        Format 2A: DCI Format 2A is used for the transmission of resource assignments for PDSCH for open-loop MIMO operation. The information transmitted is the same as for Format 2, except that if the eNodeB has two transmit antenna ports, there is no precoding information, and for four antenna ports two bits are used to indicate the transmission rank (transmission mode 3).        Format 2B: Introduced in Release 9 and is used for the transmission of resource assignments for PDSCH for dual-layer beamforming (transmission mode 8).        Format 2C: Introduced in Release 10 and is used for the transmission of resource assignments for PDSCH for closed-loop single-user or multi-user MIMO operation with up to 8 layers (transmission mode 9).        Format 2D: introduced in Release 11 and used for up to 8 layer transmissions; mainly used for COMP (Cooperative Multipoint) (transmission mode 10)        Format 3 and 3A: DCI formats 3 and 3A are used for the transmission of power control commands for PUCCH and PUSCH with 2-bit or 1-bit power adjustments respectively. These DCI formats contain individual power control commands for a group of UEs.        Format 4: DCI format 4 is used for the scheduling of the PUSCH, using closed-loop spatial multiplexing transmissions in uplink transmission mode 2.        Format 5: DCI format 5 is used for the scheduling of the PSCCH (Physical Sidelink Control Channel), and also contains several SCI format 0 fields used for the scheduling of the PSSCH (Physical Sidelink Shared Control Channel). If the number of information bits in DCI format 5 mapped onto a given search space is less than the payload size of format 0 for scheduling the same serving cell, zeros shall be appended to format 5 until the payload size equals that of format 0 including any padding bits appended to format 0.        
The 3GPP technical standard TS 35.212, current version 12.4.0, defines in subclause 5.4.3, incorporated herein by reference, control information for the sidelink; for detailed information on sidelink see later.
SCI may transport sidelink scheduling information for one destination ID. SCI Format 0 is defined for use for the scheduling of the PSSCH. The following information is transmitted by means of the SCI format 0:                Frequency hopping flag—1 bit.        Resource block assignment and hopping resource allocation        Time resource pattern—7 bits.        Modulation and coding scheme—5 bits        Timing advance indication—11 bits        Group destination ID—8 bits        
Logical Channel Prioritization, LCP, Procedure
For the uplink the process by which a UE creates a MAC PDU to transmit using the allocated radio resources is fully standardized; this is designed to ensure that the UE satisfies the QoS of each configured radio bearer in a way which is optimal and consistent between different UE implementations. Based on the uplink transmission resource grant message signaled on the PDCCH, the UE has to decide on the amount of data for each logical channel to be included in the new MAC and, if necessary, also to allocate space for a MAC Control Element.
In constructing a MAC PDU with data from multiple logical channels, the simplest and most intuitive method is the absolute priority-based method, where the MAC PDU space is allocated to logical channels in decreasing order of logical channel priority. This is, data from the highest-priority logical channel are served first in the MAC PDU, followed by data from the next highest-priority logical channel, continuing until the MAC PDU space runs out. Although the absolute priority-based method is quite simple in terms of UE implementation, it sometimes leads to starvation of data from low-priority logical channels; starvation means that the data from the low-priority logical channels cannot be transmitted because the data from high-priority logical channels take up all the MAC PDU space.
In LTE, a Prioritized Bit Rate (PBR) is defined for each logical channel so as to transmit data in the order of importance but also to avoid starvation of data with lower priority. The PBR is the minimum data rate guaranteed for the logical channel. Even if the logical channel has low priority, at least a small amount of MAC PDU space is allocated to guarantee the PBR. Thus, the starvation problem can be avoided by using the PBR.
Constructing a MAC PDU with PBR consists of two rounds. In the first round, each logical channel is served in a decreasing order of logical channel priority, but the amount of data from each logical channel included in the MAC PDU is initially limited to the amount corresponding to the configured PBR value of the logical channel. After all logical channels have been served up to their PBR values, if there is room left in the MAC PDU, the second round is performed. In the second round, each logical channel is served again in decreasing order of priority. The major difference for the second round compared to the first round is that each logical channel of lower priority can be allocated with MAC PDU space only if all logical channels of higher priority have no more data to transmit.
A MAC PDU may include not only the MAC SDUs from each configured logical channel but also a MAC CE. Except for a Padding BSR, the MAC CE has a higher priority than a MAC SDU from the logical channels because it controls the operation of the MAC layer. Thus, when a MAC PDU is composed, the MAC CE, if it exists, is the first to be included, and the remaining space is used for MAC SDUs from the logical channels. Then, if additional space is left and it is large enough to include a BSR, a Padding BSR is triggered and included in the MAC PDU. The Logical Channel Prioritization (LCP) procedure is applied every time a new transmission is performed.
The Logical Channel Prioritization is standardized e.g., in 3GPP TS 36.321 (current version v12.5.0) in subclause 5.4.3.1 incorporated herein by reference.
RRC controls the scheduling of uplink data by signaling for each logical channel:                priority where an increasing priority value indicates a lower priority level,        prioritisedBitRate which sets the Prioritized Bit Rate (PBR),        bucketSizeDuration which sets the Bucket Size Duration (BSD).        
The UE shall maintain a variable Bj for each logical channel j. Bj shall be initialized to zero when the related logical channel is established, and incremented by the product PBR×TTI duration for each TTI, where PBR is the Prioritized Bit Rate of logical channel j. However, the value of Bj can never exceed the bucket size, and if the value of Bj is larger than the bucket size of logical channel j, it shall be set to the bucket size. The bucket size of a logical channel is equal to PBR×BSD, where PBR and BSD are configured by upper layers.
LTE Device to Device (D2D) Proximity Services (ProSe)
Proximity-based applications and services represent an emerging social-technological trend. The identified areas include services related to commercial services and Public Safety that would be of interest to operators and users. The introduction of a Proximity Services (ProSe) capability in LTE would allow the 3GPP industry to serve this developing market and will, at the same time, serve the urgent needs of several Public Safety communities that are jointly committed to LTE.
Device-to-Device (D2D) communication is a technology component for LTE-Rel.12. The Device-to-Device (D2D) communication technology allows D2D as an underlay to the cellular network to increase the spectral efficiency. For example, if the cellular network is LTE, all data carrying physical channels use SC-FDMA for D2D signaling. In D2D communications, user equipments transmit data signals to each other over a direct link using the cellular resources instead of through the radio base station. Throughout the invention the terms “D2D”, “ProSe” and “sidelink” are interchangeable.
D2D Communication in LTE
The D2D communication in LTE is focusing on two areas: Discovery and Communication.
ProSe (Proximity based Services) Direct Discovery is defined as the procedure used by the ProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximity using E-UTRA direct radio signals via the PC5 interface. FIG. 3 schematically illustrates a PC5 interface for device-to-device direct discovery. FIG. 4 schematically illustrates a Radio Protocol Stack (AS) for ProSe Direct Discovery.
In D2D communication UEs transmit data signals to each other over a direct link using the cellular resources instead of through the base station (BS). D2D users communicate directly while remaining controlled under the BS, i.e., at least when being in coverage of an eNB. Therefore, D2D can improve system performances by reusing cellular resources.
It is assumed that D2D operates in the uplink LTE spectrum (in the case of FDD) or uplink sub-frames of the cell giving coverage (in case of TDD, except when out of coverage). Furthermore, D2D transmission/reception does not use full duplex on a given carrier. From individual UE perspective, on a given carrier D2D signal reception and LTE uplink transmission do not use full duplex, i.e., no simultaneous D2D signal reception and LTE UL transmission is possible.
In D2D communication, when one particular UE1 has a role of transmission (transmitting user equipment or transmitting terminal), UE1 sends data, and another UE2 (receiving user equipment) receives it. UE1 and UE2 can change their transmission and reception role. The transmission from UE1 can be received by one or more UEs like UE2.
With respect to the User plane protocols, in the following part of the agreement from D2D communication perspective is given (see also 3GPP TR 36.843 current version 12.0.1 section 9.2.2, incorporated herein by reference):                PDCP:                    1:M D2D broadcast communication data (i.e., IP packets) should be handled as the normal user-plane data.            Header-compression/decompression in PDCP is applicable for 1:M D2D broadcast communication.                            U-Mode is used for header compression in PDCP for D2D broadcast operation for public safety;                                                RLC:                    RLC UM is used for 1:M D2D broadcast communication.            Segmentation and Re-assembly is supported on L2 by RLC UM.            A receiving UE needs to maintain at least one RLC UM entity per transmitting peer UE.            An RLC UM receiver entity does not need to be configured prior to reception of the first RLC UM data unit.            So far no need has been identified for RLC AM or RLC TM for D2D communication for user plane data transmission.                        MAC:                    No HARQ feedback is assumed for 1:M D2D broadcast communication            The receiving UE needs to know a source ID in order to identify the receiver RLC UM entity.            The MAC header comprises a L2 target ID which allows filtering out packets at MAC layer.            The L2 target ID may be a broadcast, group cast or unicast address.                            L2 Groupcast/Unicast: A L2 target ID carried in the MAC header would allow discarding a received RLC UM PDU even before delivering it to the RLC receiver entity.                L2 Broadcast: A receiving UE would process all received RLC PDUs from all transmitters and aim to re-assemble and deliver IP packets to upper layers.                                    MAC sub header contains LCIDs (to differentiate multiple logical channels).            At least Multiplexing/de-multiplexing, priority handling and padding are useful for D2D.                        
ProSe Direct Communication Related Identities
3GPP TS 36.300 current version 12.5.0 defines in subclause 8.3 the following identities to use for ProSe Direct Communication:                SL-RNTI: (SideLink-Radio Network Temporary Identifier) Unique identification used for ProSe Direct Communication Scheduling;        Source Layer-2 ID: Identifies the sender of the data in sidelink ProSe Direct Communication. The Source Layer-2 ID is 24 bits long and is used together with ProSe Layer-2 Destination ID and LCD for identification of the RLC UM entity and PDCP entity in the receiver;        Destination Layer-2 ID: Identifies the target of the data in sidelink ProSe Direct Communication. The Destination Layer-2 ID is 24 bits long and is split in the MAC layer into two bit strings:                    One bit string is the LSB part (8 bits) of Destination Layer-2 ID and forwarded to physical layer as Sidelink Control Layer-1 ID. This identifies the target of the intended data in Sidelink Control and is used for filtering of packets at the physical layer.            Second bit string is the MSB part (16 bits) of the Destination Layer-2 ID and is carried within the MAC header. This is used for filtering of packets at the MAC layer.                        
Non-Access Stratum signaling is required for group formation and to configure Source Layer-2 ID, Destination Layer-2 ID and Sidelink Control L1 ID in the UE. These identities are either provided by a higher layer or derived from identities provided by a higher layer. In case of groupcast and broadcast, the ProSe UE ID provided by the higher layer is used directly as the Source Layer-2 ID, and the ProSe Layer-2 Group ID provided by the higher layer is used directly as the Destination Layer-2 ID in the MAC layer.
Radio Resource Allocation for Proximity Services
From the perspective of a transmitting UE, a Proximity-Services-enabled UE (ProSe-enabled UE) can operate in two modes for resource allocation:
On the one hand, Mode 1 refers to the eNB-scheduled resource allocation, where the UE requests transmission resources from the eNB (or Release-10 relay node), and the eNodeB (or Release-10 relay node) in return schedules the resources for use by a UE to transmit direct data and direct control information, DCI (e.g., Scheduling Assignment). The UE needs to be RRC_CONNECTED in order to transmit data. In particular, the UE sends a D2D scheduling request (D-SR or Random Access) to the eNB followed by a buffer status report (BSR) in the usual manner (see also following chapter “Transmission procedure for D2D communication”). Based on the BSR, the eNB can determine that the UE has data for a ProSe Direct Communication transmission and can estimate the resources needed for transmission.
On the other hand, Mode 2 refers to the UE-autonomous resource selection, where a UE on its own selects resources (time and frequency) from resource pool(s) to transmit direct data and direct control information (i.e., SA). One resource pool is defined e.g., by the content of SIB18, namely by the field commTxPoolNormalCommon, this particular resource pool being broadcast in the cell and then commonly available for all UEs in the cell still in RRC_Idle state. Effectively, the eNB may define up to four different instances of said pool, respectively four resource pools for the transmission of SA messages and direct data. However, a UE shall always use the first resource pool defined in the list, even if it was configured with multiple resource pools.
As an alternative, another resource pool can be defined by the eNB and signaled in SIB18, namely by using the field commTxPoolExceptional, which can be used by the UEs in exceptional cases.
What resource allocation mode a UE is going to use is configurable by the eNB. Furthermore, what resource allocation mode a UE is going to use for D2D data communication may also depend on the RRC state, i.e., RRC_IDLE or RRC_CONNECTED, and the coverage state of the UE, i.e., in-coverage, out-of-coverage. A UE is considered in-coverage if it has a serving cell (i.e., the UE is RRC_CONNECTED or is camping on a cell in RRC_IDLE).
The following rules with respect to the resource allocation mode apply for the UE:                If the UE is out-of-coverage, it can only use Mode 2;        If the UE is in-coverage, it may use Mode 1 if the eNB configures it accordingly;        If the UE is in-coverage, it may use Mode 2 if the eNB configures it accordingly;        When there are no exceptional conditions, UE may change from Mode 1 to Mode 2 or vice-versa only if it is configured by eNB to do so. If the UE is in-coverage, it shall use only the mode indicated by eNB configuration unless one of the exceptional cases occurs;                    The UE considers itself to be in exceptional conditions e.g., while T311 or T301 is running;                        When an exceptional case occurs the UE is allowed to use Mode 2 temporarily even though it was configured to use Mode 1.        
While being in the coverage area of an E-UTRA cell, the UE shall perform ProSe Direct Communication Transmission on the UL carrier only on the resources assigned by that cell, even if resources of that carrier have been pre-configured e.g., in UICC (Universal Integrated Circuit Card).
For UEs in RRC_IDLE the eNB may select one of the following options:                The eNB may provide a Mode 2 transmission resource pool in SIB. UEs that are authorized for ProSe Direct Communication use these resources for ProSe Direct Communication in RRC_IDLE;        The eNB may indicate in SIB that it supports D2D but does not provide resources for ProSe Direct Communication. UEs need to enter RRC_CONNECTED to perform ProSe Direct Communication transmission.        
For UEs in RRC_CONNECTED:                A UE in RRC_CONNECTED that is authorized to perform ProSe Direct Communication transmission, indicates to the eNB that it wants to perform ProSe Direct Communication transmissions when it needs to perform a ProSe Direct Communication transmission;        The eNB validates whether the UE in RRC_CONNECTED is authorized for ProSe Direct Communication transmission using the UE context received from the MME;        The eNB may configure a UE in RRC_CONNECTED by dedicated signaling with a Mode-2 resource allocation transmission resource pool that may be used without constraints while the UE is RRC_CONNECTED. Alternatively, the eNB may configure a UE in RRC_CONNECTED by dedicated signaling with a Mode 2 resource allocation transmission resource pool which the UE is allowed to use only in exceptional cases and to rely on Mode 1 otherwise.        
The resource pool for Scheduling Assignment when the UE is out-of-coverage can be configured as below:                The resource pool used for reception is pre-configured.        The resource pool used for transmission is pre-configured.        
The resource pool for Scheduling Assignment when the UE is in coverage can be configured as below:                The resource pool used for reception is configured by the eNB via RRC, in dedicated or broadcast signaling.        The resource pool used for transmission is configured by the eNB via RRC if Mode 2 resource allocation is used        The SCI (Sidelink Control Information) resource pool (also referred to as Scheduling Assignment, SA, resource pool) used for transmission is not known to the UE if Mode 1 resource allocation is used.        The eNB schedules the specific resource(s) to use for Sidelink Control Information (Scheduling Assignment) transmission if Mode 1 resource allocation is used. The specific resource assigned by the eNB is within the resource pool for reception of SCI that is provided to the UE.        
FIG. 5 illustrates the use of transmission/reception resources for overlay (LTE) and underlay (D2D) system.
Basically, the eNodeB controls whether UE may apply the Mode 1 or Mode 2 transmission. Once the UE knows its resources where it can transmit (or receive) D2D communication, in the current state-of-the-art, it uses the corresponding resources only for the corresponding transmission/reception. For example, in FIG. 5 the D2D subframes will only be used to receive or transmit the D2D signals. Since the UE as a D2D device would operate in Half Duplex mode, it can either receive or transmit the D2D signals at any point of time. Similarly, the other subframes illustrated in FIG. 5 can be used for LTE (overlay) transmissions and/or reception.
Transmission Procedure for D2D Communication
The D2D data transmission procedure differs depending on the resource allocation mode. As described above for Mode 1, the eNB explicitly schedules the resources for the Scheduling Assignment and the D2D data communication after a corresponding request from the UE. Particularly, the UE may be informed by the eNB that D2D communication is generally allowed, but that no Mode 2 resources (i.e., resource pool) are provided; this may be done e.g., with the exchange of the D2D communication Interest Indication by the UE and the corresponding response, D2D Communication Response, where the corresponding exemplary ProseCommConfig information element mentioned above would not include the commTxPoolNormalCommon, meaning that a UE that wants to start direct communication involving transmissions has to request E-UTRAN to assign resources for each individual transmission. Thus, in such a case, the UE has to request the resources for each individual transmission, and in the following the different steps of the request/grant procedure are exemplarily listed for this Mode 1 resource allocation:                Step 1: UE sends SR (Scheduling Request) to eNB via PUCCH;        Step 2: eNB grants UL resource (for UE to send BSR) via PDCCH, scrambled by C-RNTI;        Step 3: UE sends D2D BSR indicating the buffer status via PUSCH;        Step 4: eNB grants D2D resource (for UE to send data) via PDCCH, scrambled by SL-RNTI.        Step 5: D2D Tx UE transmits SA/D2D data according to grant received in step 4.        
A Scheduling Assignment (SA), also termed SCI (Sidelink Control Information) is a compact (low-payload) message containing control information, e.g., pointer(s) to time-frequency resources, modulation and coding scheme and Group Destination ID for the corresponding D2D data transmission. An SCI transports the sidelink scheduling information for one (ProSE) destination ID. The content of the SA (SCI) is basically in accordance with the grant received in Step 4 above. The D2D grant and SA content (i.e., SCI content) are defined in the 3GPP technical standard TS 36.212, current version 12.4.0, subclause 5.4.3, incorporated herein by reference, defining in particular the SCI format 0 as mentioned before in this background section.
On the other hand, for Mode 2 resource allocation, above steps 1-4 are basically not necessary, and the UE autonomously selects resources for the SA and D2D data transmission from the transmission resource pool(s) configured and provided by the eNB.
FIG. 6 exemplarily illustrates the transmission of the Scheduling Assignment and the D2D data for two UEs, UE-A and UE-B, where the resources for sending the scheduling assignments are periodic, and the resources used for the D2D data transmission are indicated by the corresponding Scheduling Assignment.
FIG. 7 illustrates the D2D communication timing for Mode 2, autonomous scheduling, during one SA/data period, also known as SC period, Sidelink Control period. FIG. 8 illustrates the D2D communication timing for Mode 1, eNB-scheduled allocation during one SA/data period. A SC period is the time period consisting of transmission of a Scheduling Assignment and its corresponding data.
As can be seen from FIG. 7, the UE transmits after an SA-offset time, a Scheduling Assignment using the transmission pool resources for scheduling assignments for Mode 2, SA_Mode2_Tx_pool. The 1st transmission of the SA is followed e.g., by three retransmissions of the same SA message. Then, the UE starts the D2D data transmission, i.e., more in particular using a time resource pattern of transmission, i.e., T-RPT bitmap/pattern, at some configured offset (Mode2data_offset) after the first subframe of the SA resource pool (given by the SA_offset).
One D2D data transmission of a MAC PDU consists of its 1st transmissions and several retransmissions. For the illustration of FIG. 7 (and of FIG. 8) it is assumed that three retransmissions are performed (i.e., 2nd, 3rd, and 4th transmission of the same MAC PDU). The Mode2 T-RPT Bitmap (time resource pattern of transmission, T-RPT) basically defines the timing of the MAC PDU transmission (1st transmission) and its retransmissions (2nd, 3rd, and 4th transmission).
During one SA/data period, the UE can transmit multiple transport blocks (only one per subframe (TTI), i.e., one after the other), however to only one ProSe destination group. Also the retransmissions of one transport block must be finished before the first transmission of the next transport block starts, i.e., only one HARQ process is used for the transmission of the multiple transport blocks.
As apparent from FIG. 8, for the eNB-scheduled resource allocation mode (Mode 1), the D2D data transmission, i.e., more in particular the T-RPT pattern/bitmap, starts in the next UL subframe after the last SA transmission repetition in the SA resource pool. As explained already for FIG. 7, the Mode1 T-RPT Bitmap (time resource pattern of transmission, T-RPT) basically defines the timing of the MAC PDU transmission (1st transmission) and its retransmissions (2nd, 3rd, and 4th transmission).
ProSe Network Architecture and ProSe Entities
FIG. 9 illustrates a high-level exemplary architecture for a non-roaming case, including different ProSe applications in the respective UEs A and B, as well as a ProSe Application Server and ProSe function in the network. The example architecture of FIG. 9 is taken from TS 23.303 v.12.4.0 chapter 4.2 “Architectural Reference Model” incorporated herein by reference.
The functional entities are presented and explained in detail in TS 23.303, subclause 4.4 titled “Functional Entities” which is incorporated herein by reference. The ProSe function is the logical function that is used for network-related actions required for ProSe and plays different roles for each of the features of ProSe. The ProSe function is part of the 3GPP's evolved packet core, EPC, and provides all relevant network services like authorization, authentication, data handling etc. related to proximity services.
For ProSe direct discovery and communication, the UE may obtain a specific ProSe UE identity, other configuration information, as well as authorization from the ProSe function over the PC3 reference point. There can be multiple ProSe functions deployed in the network, although for ease of illustration a single ProSe function is presented. The ProSe function consists of three main sub-functions that perform different roles depending on the ProSe feature: Direct Provision Function (DPF), Direct Discovery Name Management Function, and EPC-level Discovery Function. The DPF is used to provision the UE with necessary parameters in order to use ProSe Direct Discovery and ProSe Direct Communication.
The term “UE” used in said connection refers to a ProSe-enabled UE supporting ProSe functionality, such as:                Exchange of ProSe control information between ProSe-enabled UE and the ProSe Function over PC3 reference point.        Procedures for open ProSe Direct Discovery of other ProSe-enabled UEs over PC5 reference point.        Procedures for one-to-many ProSe Direct Communication over PC5 reference point.        Procedures to act as a ProSe UE-to-Network Relay. The Remote UE communicates with the ProSe UE-to-Network Relay over PC5 reference point. The ProSe UE-to Network Relay uses layer-3 packet forwarding.        Exchange of control information between ProSe UEs over PC5 reference point, e.g., for UE-to-Network Relay detection and ProSe Direct Discovery.        Exchange of ProSe control information between another ProSe-enabled UE and the ProSe Function over PC3 reference point. In the ProSe UE-to-Network Relay case the Remote UE will send this control information over PC5 user plane to be relayed over the LTE-Uu interface towards the ProSe Function.        Configuration of parameters (e.g., including IP addresses, ProSe Layer-2 Group IDs, Group security material, radio resource parameters). These parameters can be pre-configured in the UE, or, if in coverage, provisioned by signaling over the PC3 reference point to the ProSe Function in the network.        
The ProSe Application Server supports the Storage of EPC ProSe User IDs, and ProSe Function IDs, and the mapping of Application Layer User IDs and EPC ProSe User IDs. The ProSe Application Server (AS) is an entity outside the scope of 3GPP. The ProSe application in the UE communicates with the ProSe AS via the application-layer reference point PC1. The ProSe AS is connected to the 3GPP network via PC2 reference point.
UE Coverage States for D2D
As already mentioned before, the resource allocation method for D2D communication depends, apart from the RRC state, i.e., RRC_IDLE and RRC_CONNECTED, also on a coverage state of the UE, i.e., in-coverage, out-of-coverage. A UE is considered in-coverage if it has a serving cell (i.e., the UE is RRC_CONNECTED or is camping on a cell in RRC_IDLE).
The two coverage states mentioned so far, i.e., in-coverage (IC) and out-of-coverage (OOC), are further distinguished into sub-states for D2D. FIG. 10 shows the four different states a D2D UE can be associated to, which can be summarized as follows:                State 1: UE1 has uplink and downlink coverage. In this state the network controls each D2D communication session. Furthermore, the network configures whether UE1 should use resource allocation Mode 1 or Mode 2.        State 2: UE2 has downlink but no uplink coverage, i.e., only DL coverage. The network broadcasts a (contention-based) resource pool. In this state the transmitting UE selects the resources used for SA and data from a resource pool configured by the network; resource allocation is only possible according to Mode 2 for D2D communication in such a state.        State 3: UE3 has no uplink and downlink coverage, accordingly, the UE3 is considered as out-of-coverage (OOC). However, UE3 is in coverage of some UEs which themselves (e.g., UE1) are in coverage of the cell, i.e., those UEs can be also referred as CP-relay UEs. Therefore, the area of the state-3 UEs in FIG. 10 can be denoted as CP UE-relay coverage area. UEs in this state 3 are also referred to as OOC-state-3 UEs. In this state the UEs receive some cell specific information which is sent by the eNB (SIB) and forwarded by the CP UE-relay UEs in the coverage of the cell via PD2DSCH to the OOC-state-3 UEs. A (contention-based) network-controlled resource pool is signaled by PD2DSCH.        State 4: UE4 is out of coverage and does not receive PD2DSCH from other UEs which are in the coverage of a cell. In this state, which is also referred to as state-4 OOC, the transmitting UE selects the resources used for the data transmission from a (contention-based) pre-configured resource pool.        
The reason for distinguishing between state-3 OOC and state-4 OOC is mainly to avoid potential interference between D2D transmissions from out-of coverage devices and legacy E-UTRA transmissions. In general, D2D-capable UEs will have preconfigured resource pool(s) for transmission of D2D SAs and data for use while out of coverage. If these out-of-coverage UEs transmit on these preconfigured resource pools at the cell boundaries, then, interference between the D2D transmissions and in-coverage legacy transmissions can have a negative impact on communications within the cell.
If D2D-enabled UEs within coverage forward the D2D resource pool configuration to those out-of-coverage devices near the cell boundary, then, the out-of-coverage UEs can restrict their transmissions to the resources specified by the eNode B and therefore minimize interference with legacy transmissions in coverage. Thus, RANI has introduced a mechanism where in-coverage UEs are forwarding resource pool information and other D2D related configurations to those devices just outside the coverage area (state-3 UEs).
The Physical D2D synchronization channel (PD2DSCH) is used to carry this information about in-coverage D2D resource pools to the UEs in network proximity, so that resource pools within network proximity are aligned.
LCP Procedure for D2D, Sidelink Logical Channels
The LCP procedure for D2D will be different from the above-presented LCP procedure for “normal” LTE data. The following information is taken from TS 36.321, version 12.5.0, subclause 5.14.1.3.1 describing LCP for ProSe; which is incorporated herewith in its entirety by reference.
The UE shall perform the following Logical Channel Prioritization procedure when a new transmission is performed:                The UE (e.g., MAC entity) shall allocate resources to the sidelink logical channels according to the following rules:                    the UE should not segment an RLC SDU (or partially transmitted SDU) if the whole SDU (or partially transmitted SDU) fits into the remaining resources;            if the UE segments an RLC SDU from the sidelink logical channel, it shall maximize the size of the segment to fill the grant as much as possible;            the UE should maximize the transmission of data.            if the UE is given an sidelink grant size that is equal to or larger than 10 bytes while having data available for transmission, the UE shall not transmit only padding.                        
NOTE: The rules above imply that the order by which the sidelink logical channels are served is left for UE implementation.
Generally, for one PDU, a MAC entity shall consider only logical channels with the same Source Layer-2ID—Destination Layer 2 ID pairs, i.e., for one PDU, the MAC entity in the UE shall consider only logical channels of the same ProSe destination group, i.e., having a same destination group ID. The UE selects a ProSe destination group during the LCP procedure. Furthermore, in Rel-12 during one SA/data period the D2D transmitting UE can only transmit data to one ProSe destination group.
All D2D (sidelink) logical channels, e.g., STCH, Sidelink Traffic CHannel, are allocated to the same logical channel group (LCG), namely with LCGID set to ‘11’ (see subclause 5.14.1.4 “Buffer Status Reporting” of TS 36.321 version 12.5.0). In Rel-12 there is no prioritization mechanism for D2D (sidelink) logical channels/groups. Essentially, all sidelink logical channels have the same priority from UE point of view, i.e., the order by which the sidelink logical channels are served is left for UE implementation.
For Rel-13 a more advanced prioritization mechanism is considered where each sidelink logical channel is associated with a logical channel priority, also referred to as PPPP (ProSe per packet priority). Based on this logical channel priority the UE selects the ProSe destination group for a given sidelink grant, i.e., highest priority logical channel determines the ProSe destination group, and further allocates resources to the logical channels belonging to the selected ProSe destination group (in decreasing priority order).
For illustration purposes only, the following exemplary scenario is considered where three ProSe logical channels, LCH#1, LCH#2, and LCH#3, are set up in the user equipment, and all three are associated with the same ProSe LCG (e.g., “11”). It is exemplarily assumed that LCH#1 and LCH#2 are assigned to ProSe destination group 1, and LCH#3 is assigned to ProSe destination group 2. This is depicted in FIG. 12.
Buffer Status Reporting for ProSe
Also the buffer status reporting is adapted to ProSe, and at present is defined in TS 36.321 in its version 12.5.0, subclause 5.14.1.4 “Buffer Status Reporting” incorporated herein by reference for Rel-12.
The (D2D) sidelink Buffer Status Reporting procedure is used to provide the serving eNB with information about the amount of sidelink data available for transmission in the sidelink buffers of the UE. RRC controls sidelink BSR reporting by configuring the two timers Periodic-ProseBSR-Timer and RetxProseBSR-Timer. Each sidelink logical channel (STCH) is allocated to an LCG with LCGID set to “11” and belongs to a ProSe Destination group.
A sidelink Buffer Status Report (BSR) shall be triggered if some particular events occurs, as specified in detail in TS 36.321, subclause 5.14.1.4.
Furthermore, TS 36.321 in its version 12.5.0, subclause 6.1.3.1a, incorporated herein by reference, defines the ProSe BSR MAC Control Elements and its corresponding content as follows. The ProSe Buffer Status Report (BSR) MAC control element consists of one group index field, one LCG ID field and one corresponding Buffer Size field per reported D2D destination group. In more detail, for each included ProSe destination group, the following fields are defined:                Group index: The group index field identifies the ProSe destination group. The length of this field is 4 bits. The value is set to the index of the destination identity reported in destinationInfoList;        LCG ID: The Logical Channel Group ID field identifies the group of logical channel(s) which buffer status is being reported. The length of the field is 2 bits and it is set to “11”;        Buffer Size: The Buffer Size field identifies the total amount of data available across all logical channels of a ProSe Destination group after all MAC PDUs for the TTI have been built. The amount of data is indicated in number of bytes        R: Reserved bit, set to “0”.        
FIG. 11 shows the ProSe BSR MAC control element for even N (number of ProSe destination groups), taken from TS 36.321 subclause 6.1.3.1a.
As has been explained above, the transmission scheme for device-to-device communication is different from the normal LTE scheme, including the use of ProSe destination groups to identify the possible content of the data. Some of currently-defined mechanisms are rather inefficient.